1. Field of the Invention
This invention relates to a prosthetic bearing element and to a process for making such an element. More particularly it relates to the use of a polyurethane bearing component which results in the formation of a fluid film bearing between the metal joint part and the bearing surface.
2. Description of the Prior Art
Improved wear of bearing surfaces such as polyethylene acetabular components in hips, tibial component in knees, and other prosthetic bearing inserts is a major goal for the success of artificial joints. Orthopaedic devices for joint construction and reconstruction can comprise polyethylene bearings on metal, ceramic on polyethylene, and metal on metal. Polyethylene bearings are often held in a metal shell, for example in hip prosthesis or a tibial tray in knee prosthetics. Conventionally bearing liners made from ultra-high molecular weight polyethylene are fixed via a snap fit into the metal backing.
The possible use of elastomers in bearings to provide fluid film lubrication at low velocity, low viscosity lubricant has been published in laboratory studies which show that such "soft" bearings provide lower coefficient of friction compared with standard polyethylene versus metal bearings. Such a material is polyurethane. This fluid film bearing mimics the natural lubrication of the joint where synovial fluid lubricates the bone cartilage interface. Because of the fluid film lubrication, it is less likely that the two bearings touch during use and thus wear is lower.
It is however difficult to apply this concept to artificial joints for a number of reasons. One difficulty lies in selecting a suitable elastomer such as polyurethane which will not be degraded or rejected by the human body so care must be taken in selecting the polyurethane materials used. As mentioned above, polyethylene bearings are usually held by physical means in, for example, a metal shell but it is not possible to use this approach with polyurethane because as its stiffness is so much lower than the metal it would be possible under load to be extruded from the metal shell or easily moved relative to the shell causing damage. Moreover, the lower stiffness of the bearing leads to greater differential interface stain between the shell and the liner.
Because of the above difficulties the applicants have tried several different approaches. Initially, for ease of use and conventional appearance, it was attempted to bond a polyurethane liner into a polyethylene backing, similar to pre-existing polyethylene components. This could then be held in a metal shell or cemented depending on the surgeon's preference.